OAKLAND, Calif. (AP) - For weeks the protests had waned, with only a smattering of people taking to Oakland's streets for occasional marches that bore little resemblance to the headline-grabbing Occupy demonstrations of last fall.
Then came Saturday, which started peacefully enough - a midday rally at City Hall and a march. But hours later, the scene near downtown Oakland had dramatically deteriorated: clashes punctuated by rock and bottle throwing by protesters and volleys of tear gas from police, and a City Hall break-in that left glass cases smashed, graffiti spray-painted on walls and an American flag burned.
More than 400 people were arrested on charges ranging from failure to disperse to vandalism, a police said. At least three officers and one protester were injured.
On Sunday, Oakland officials vowed to be ready if Occupy protesters try to mount another large-scale demonstration. Protesters, meanwhile, decried Saturday's police tactics as illegal and threatened to sue.
Mayor Jean Quan personally inspected damage caused by dozens of people who broke into City Hall. She said she wants a court order to keep Occupy protesters who have been arrested several times out of Oakland, which has been hit repeatedly by demonstrations that have cost the financially troubled city about $5 million.
Quan called on the loosely organized movement to "stop using Oakland as its playground."
"People in the community and people in the Occupy movement have to stop making excuses for this behavior," she said.
Saturday's protests - the most turbulent since Oakland police forcefully dismantled an Occupy encampment in November - came just days after the group announced a new round of actions. The group said it planned to use a vacant building as a social center and political hub and threatened to try to shut down the Port of Oakland for a third time, occupy the airport and take over City Hall.
After the mass arrests, the Occupy Oakland Media Committee criticized the police's conduct, saying that most of the arrests were made illegally because police failed to allow protesters to disperse, and they threatened legal action.
"Contrary to their own policy, the OPD gave no option of leaving or instruction on how to depart. These arrests are completely illegal, and this will probably result in another class action lawsuit against the OPD," a release from the group said.
Deputy Police Chief Jeff Israel told reporters late Saturday that protesters gathered unlawfully and police gave them multiple verbal warnings to disband.
"These people gathered with the intent of unlawfully entering into a building that does not belong to them and assaulting the police," Israel said. "It was not a peaceful group."
Social activism and civic unrest have long marked Oakland, a rough-edged city of nearly 400,000 across the bay from San Francisco. Beset by poverty, crime and a decades-long tense relationship between the police and the community, its streets have seen clashes between officers and protesters, including anti-draft protests in the 1960s that spilled into town from neighboring Berkeley.
Police maintained a guard at City Hall overnight, and dozens of officers were on the scene Sunday.
"They were never able to occupy a building outside of City Hall," Interim Police Chief Howard Jordan said Sunday. "We suspect they will try to go to the convention center again. They will get not get in."
Quan hopes prosecutors will seek a stay-away order against protesters who have been arrested multiple times.
"It appears that most of them constantly come from outside of Oakland," she said. "I think a lot of the young people who come to these demonstrations think they're being revolutionary when they're really hurting the people they claim that they are representing."
Saturday's events began midday when a group assembled outside City Hall and marched through the streets, disrupting traffic as they threatened to take over a vacant convention center.